Magnetic cores, such as toroidal-shaped cores, have been previously used for temperature sensing. The prior art methods of temperature sensing utilized transition characteristics of the magnetic core such as the Curie temperature transition and/or first order transitions such as those described in U.S. Pat. No. 3,534,306; issued on Oct. 13, 1970, in the name of Watrous et al. Prior temperature sensing devices of this type relied on the fact that at a certain temperature a drastic change of the magnetic characteristics of the core would occur. Thus, if a wire were wound around the core to form an inductance element, the inductance of the element would change drastically when the predetermined temperature was reached. This required specific core materials that were specially formulated and carefully controlled in order to provide thhe desired rapid transition at the exact temperature that was desired. A different specially manufactured magnetic core would then have to be substituted in the sensor in order to sense another temperature.
The sensing device of the present invention, by contrast, does not depend upon any rapid change of inductance state of a magnetic core. In the present sensing device, the inductance of the magnetic elements varies in a gradual manner until the inductance of both elements is approximately equal at a predetermined temperature which is then sensed by the sensing circuit. The advantage of this approach over the prior art devices is that by changing the inductance of the element by changing the number of windings coupled to it, the cross-over point where the two inductances are equal may be changed so that the temperature sensor may be used over wide range temperatures.
The present invention is achieved by coupling the two inductively wound elements having different inductance vs. temperature characteristics into a four-arm A.C. inductance bridge circuit having two terminals that are connected to conventional null detector. When the inductance vs. temperature characteristics of the two elements cross at a predetermined temperature, the inductances are equal; and the null detector indicates that the desired temperature has been reached. Although two magnetic cores have been connected in series to achieve temperature compensation, as is shown in U.S. Pat. No. 3,824,502; issued on July 16, 1974, to Bardash et al., the utilization of two series connected magnetic elements that have different temperature characteristics for sensing temperatures over a relatively large range of temperatures without a transition change of the magnetic state of the element was not achieved by the device of the Bardash et al patent.
In a co-pending U.S. patent application, Ser. No. 533,364, entitled Two-Core Magnetic Temperature Sensor, filed in the name of Edward F. Sidor, and assigned to the assignee of the present invention, a temperature sensing circuit utilizing two magnetic cores was described. In this circuit, the two cores were connected into a bridge circuit with two other impedances and were coupled to a sensing circuit, such as a null detector, in order to sense the temperature vs. inductance crossover point at which the inductance of the two cores became equal. As noted above, one advantage of this type of device was that by changing the inductance of the device, the temperature cross-over point could easily be varied; and the temperature sensor could be used over a wide range of temperatures. The prior application contemplated change of inductance by means of varying the number of windings wound on the cores of the two sensing elements. This approach, however, was time consuming since it required disassembly of the sensing unit.
The present invention, by contrast, allows for the adjustment of the temperature cross-over point by movement of one or more permanent magnets which are positioned adjacent the magnetic sensing elements, so that by adjustment of the position of the movable magnets, the permeability of the magnetic elements may be adjusted in order to vary the temperature cross-over point without disassembly of the circuit.
A further advantageous feature of the present invention is that either toroidal-shaped cores or elongated tubular magnetic elements, in which the winding consists of the wire that passes substantially along the axis of the tubular element, may be employed. By making the core elongated and tubular in shape, and by making the length of the permanent magnets so that they are somewhat shorter than the length of the tubular elements, a more precise control is achieved because the amount of saturation of the tubular elements can be closely controlled by positioning of the permanent magnet. This is accomplished by selectively, magnetically saturating a predetermined portion of the elongated magnetic elements. Similar magnetic elements had been previously proposed for use as positional transducers rather than temperature sensors. U.S. patent application Ser. No. 518,310 filed Oct. 29, 1974, in the name of Victor M. Bernin and assigned to the assignee of the present invention shows the use of such elements for a positional sensor.
In addition to employment of a single pair of magnetic elements in a two-arm active bridge circuit, two pairs of magnetic elements may be connected to form a four-arm active bridge circuit which is twice as sensitive as a two-arm active bridge circuit. A similar four-arm active bridge circuit was described in U.S. patent application Ser. No. 507,828 filed Sep. 20, 1974, in the name of Edward F. Sidor and assigned to the assignee of the present invention. In the Sidor U.S. patent application Ser. No. 507,828, however, the magnetic elements were all formed of the same magnetic materials; and the circuit was used in an angular velocity sensor and not a temperature sensor in which the elements of each pair are constructed of different magnetic materials.
In one version of the present invention, a synchronous detector circuit is used. A synchronous detector circuit is also disclosed in U.S. patent application Ser. No. 535,475 filed Dec. 13, 1974, in the name of Edward F. Sidor and Rand J. Eikelberger, which is also assigned to the assignee of the present invention. However, in the Sidor et al application, the synchronous detector was used in a circuit which sensed the Curie transition temperature of two cores so as to provide a temperature hysteresis function. In the present application, the sensing circuit is used with magnetic elements that do not undergo a sharp magnetic transition change, but instead the continuous inductance vs. temperature characteristics of the elements intersect the temperature to be sensed.